In this study, we investigate CO2 capture properties of semiclathrate hydrates formed with tetra-n- butylammonium (TBA) Br (TBAB), TBA Cl (TBAC), tetra-n-butylphosphonium (TBP) Br (TBPB) or TBP Cl (TBPC) from H-2 + CO2 mixed gas. Parametric tests found that all of these hydrates capture almost only CO2. Captured CO2 amounts basically increased with increase of aqueous concentration of ionic substances, while irregular CO2 capture was observed with a TBAB aqueous solution of which concentration was 0.3 in mass fraction. The data with this solution can be divided into two groups: CO2 capture amount of one group is three times higher than the other. It was found that this polymorphic phase behavior for TBAB hydrate was caused by a slight difference in subcooling temperature. TBAC hydrates captured the least CO2 among the presently used hydrates possibly due to its tetragonal hydrate structure which has relatively small gas capacity. Because TBPC hydrates showed unpredictable behavior of CO2 capture, it was suggested that unique hydrate phase formation with TBPC under H-2 + CO2 gas pressure. Among these four salts, TBPB was found to be suitable for H-2/CO2 gas separation due to their superior gas capture performance and simple phase behavior. (C) 2021 Elsevier Ltd. All rights reserved.